futex: Provide distinct return value when owner is exiting

attach_to_pi_owner() returns -EAGAIN for various cases:

 - Owner task is exiting
 - Futex value has changed

The caller drops the held locks (hash bucket, mmap_sem) and retries the
operation. In case of the owner task exiting this can result in a live
lock.

As a preparatory step for seperating those cases, provide a distinct return
value (EBUSY) for the owner exiting case.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.935606117@linutronix.de

diff --git a/kernel/futex.c b/kernel/futex.c
index 46a81e611065e7fda420191042779e647c4ea8ad..4f9d7a4b6dbf0cd87491018403623eefd805684b 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1182,11 +1182,11 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,
        u32 uval2;
 
        /*
-        * If the futex exit state is not yet FUTEX_STATE_DEAD, wait
-        * for it to finish.
+        * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
+        * caller that the alleged owner is busy.
         */
        if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
-               return -EAGAIN;
+               return -EBUSY;
 
        /*
         * Reread the user space value to handle the following situation:
@@ -2092,12 +2092,13 @@ retry_private:
                        if (!ret)
                                goto retry;
                        goto out;
+               case -EBUSY:
                case -EAGAIN:
                        /*
                         * Two reasons for this:
-                        * - Owner is exiting and we just wait for the
+                        * - EBUSY: Owner is exiting and we just wait for the
                         *   exit to complete.
-                        * - The user space value changed.
+                        * - EAGAIN: The user space value changed.
                         */
                        double_unlock_hb(hb1, hb2);
                        hb_waiters_dec(hb2);
@@ -2843,12 +2844,13 @@ retry_private:
                        goto out_unlock_put_key;
                case -EFAULT:
                        goto uaddr_faulted;
+               case -EBUSY:
                case -EAGAIN:
                        /*
                         * Two reasons for this:
-                        * - Task is exiting and we just wait for the
+                        * - EBUSY: Task is exiting and we just wait for the
                         *   exit to complete.
-                        * - The user space value changed.
+                        * - EAGAIN: The user space value changed.
                         */
                        queue_unlock(hb);
                        put_futex_key(&q.key);